Analysis and simulation of incident photon to current efficiency in dye sensitized solar cells
Introduction
Dye solar cells (DSC) are based on a thin layer of TiO2 nanoparticles (diameter 20 nm) covered with a monolayer of light absorbing molecules. This system is embedded in a liquid electrolyte solution containing a redox pair (Fig. 1). Essentially, a DSC is made of two main regions: the mesoporous photoanode wetted with the electrolyte that penetrates in the interstitial room, and the bulk electrolyte. This compound is sandwiched between two supporting glass layers covered by transparent conductive oxide (TCO); at the cathode is sputtered an additional layer of platinum, acting as a catalyst for the redox reaction. Despite big efforts made in finding a new suitable class of organic dyes, the best performance is obtained with Ru-based organic complexes. The standard redox pair is /, but there are promising performances obtained with ionic liquids.
Once the light reaches the dye active layer, there may occur a HOMO–LUMO electron transition (Fig. 2). Here, the excited electron can be transferred into the conduction band of the TiO2 where electrons can percolate up to the anode and be collected. The ionized dye is neutralized by the oxidation of the iodide, while the reduction of triiodide occurs at the cathode. The / couple redox reaction is the following: The driving force moving the redox ions is the concentration gradient produced under illumination. It is worth to notice that there is no chemical modification of any of the components of the cell. However, electron loss reactions occur in the cell, competing with the desired reactions (Fig. 2): electron injection can be lowered by the relaxation of the excited molecule into its ground state, or electron collection at the anode may be reduced by the neutralization of the dye with a TiO2 electron. Conduction band electrons may also be captured by the oxidized species (triiodide) of the electrolyte. The latter one, labeled as R in Fig. 2 is the most relevant dark current occurring in the cell, as it comes out comparing all the rate constants of these processes [1], [2]. In order to better understand the role played by the recombination processes in terms of cell performances, we compare simulation with experiment.
Section snippets
Model
We model our cell as a 1D pseudo-homogenous medium, where all the components (TiO2, dye, electrolyte) are intermixed. The model treats the four charge carriers (electron, iodide, triiodide and cation), coupled by the Poisson equation for the electrostatic potential. Even if the net current deriving from the cation is zero, it must be considered to achieve charge neutrality. The dynamics of all carriers is described by a set of Drift Diffusion equation coupled to continuity equation:
Application to incident photon to current efficiency
In order to gain the limitations and the range of use of the model, we applied our calculation to the experimental measurements of incident photon to current efficiency (IPCE). It measures the spectral response of the cell, containing information on quantum efficiences of the photocurrent generation (Fig. 4). In particular, IPCE can be factorized into three terms [4]: Following each step of conversion, is the efficiency of the cell in harvesting
Conclusion
We developed a 1D model to simulate the features of a DSC in the framework of TiberCAD. By varying the simulation parameters, we were able to fit an experimental – curve of a cell fabricated in our laboratories, taking into account the losses due to the sheet resistance. We focused on the role of the recombination rate in collection efficiency, calculating the generation–recombination profiles in the cell. We applied our model to the IPCE measurements, and combining the simulation results to
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